A request: "Proto-underground" recommendations?

According to a recent post, the bands Bathory, Celtic Frost, Sepultura and Sarcofago are considered proto-underground. This was defined by one poster as "more than speed metal but not quite death/black metal".

What are some worthwhile band or albums to check out for listeners whose favorite bands include all of the (in some cases only early work) aforementioned artists?

To my ears, even Morbid Angel and Possessed are closer to being "proto-underground" than, say, the first Incantation album. That is the start of death metal. Though there generally seems to be a lot less rigid classification of genres in the early days, which allowed for a wider array of influences. Not becoming too narrow-sighted (not necessarily a bad thing) or inversely adopting any and all influences just for the hell of it.

To my ears, even Morbid Angel and Possessed are closer to being "proto-underground" than, say, the first Incantation album. That is the start of death metal. Though there generally seems to be a lot less rigid classification of genres in the early days, which allowed for a wider array of influences. Not becoming too narrow-sighted (not necessarily a bad thing) or inversely adopting any and all influences just for the hell of it.

I can't disagree. Furthermore Seven Churches and Blessed are the Sick are in my top 10 metal albums of all time.